1. Field of the Invention
This invention relates to downhole tools for use in high temperature wells, and more particularly, to a high temperature bridge plug made primarily of non-metallic composite materials.
2. Description of the Prior Art
In the drilling or reworking of oil wells, a great variety of downhole tools are used. For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of the well, such as when it is desired to pump cement or other slurry down tubing and force the slurry out into a formation. It then becomes necessary to seal the tubing with respect to the well casing and to prevent the fluid pressure of the slurry from lifting the tubing out of the well. Packers and bridge plugs designed for these general purposes are well known in the art.
When it is desired to remove such downhole tools from a wellbore, it is frequently simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the packer or plug, for example, or at least the outer components thereof, out of the wellbore. Milling is a relatively slow process, but it can be used on packers or bridge plugs having relatively hard components such as erosion-resistant hard steel. One such packer is disclosed in U.S. Pat. No. 4,151,875 to Sullaway, assigned to the assignee of the present invention and sold under the trademark EZ DISPOSAL packer. Other downhole tools in addition to packers and bridge plugs may also be drilled out.
In drilling, a drill bit is used to cut and grind up the components of the downhole tool to remove it from the wellbore. This is a much faster operation than milling, but requires the tool to be made of materials which can be accommodated by the drill bit. Soft and medium hardness cast iron have been used on the pressure-bearing components, along with some brass and aluminum items. Packers of this type include the Halliburton EZ DRILL.RTM. and EZ DRILL SV.RTM. squeeze packers.
The EZ DRILL.RTM. packer and bridge plug and the EZ DRILL SV.RTM. packer are designed for fast removal from the wellbore by either rotary or cable tool drilling methods. Many of the components in these drillable packing devices are locked together to prevent their spinning while being drilled, and the harder slips are grooved so that they can be broken up in small pieces. Typically, standard "tri-cone" rotary drill bits are used.
However, drilling out iron components requires certain techniques. Ideally, the operator employs variations in rotary speed and bit weight to help break up the metal parts and re-establish bit penetration should bit penetration cease while drilling. A phenomenon known as "bit tracking" can occur, wherein the drill bit stays on one path and no longer cuts into the downhole tool. When this happens, it is necessary to pick up the bit above the drilling surface and rapidly recontact the bit with the packer or plug and apply weight while continuing rotation. This aids in breaking up the established bit pattern and helps to re-establish bit penetration. If this procedure is used, there are rarely problems. However, operators may not apply these techniques or even recognize when bit tracking has occurred. The result is that drilling times are greatly increased because the bit merely wears into the surface of the downhole tool rather than cutting into it to break it up.
While cast iron components may be necessary for the high pressures and temperatures for which they are designed, it has been determined that many wells experience pressures less than 10,000 psi and temperatures less than 425.degree. F. Thus, the heavy-duty metal construction of some previous downhole tools, such as packers and bridge plugs described above, is not necessary for many applications.
For such well conditions, tools have been designed wherein at least some of the components, including slips and pressure-bearing components, are made at least partially of non-metallic materials, such as engineering-grade plastics. Such tools are shown in U.S. Pat. Nos. 5,271,468, 5,224,540, and 5,390,737, assigned to the assignee of the present invention. These tools are sold under the trademark FAS DRILL.RTM.. The plastic components in these tools are much more easily drilled than cast iron, and new drilling methods may be employed which use alternative drill bits such as polycrystalline diamond compact bits, or the like, rather than standard tri-cone bits.
These prior tools using non-metallic components utilize two sets of slips, one on each side of the packing elements to lock the tool in the wellbore and prevent it from unsetting. This is particularly helpful in high-pressure situations to keep the tool from being undesirably moved in the wellbore. However, not all well conditions have these pressure levels, and the present invention is designed to address such less severe well conditions. The present invention utilizes a single set of slips to hold the tool in the wellbore while a plurality of ratchets keep the tool from unsetting. This results in a cost reduction compared to current plugs and packers.